1. Field of the Invention
The present invention relates to a YC separator circuit for separating a composite signal of a television video and the like into a luminance signal and a color signal. The YC separator circuit according to the present invention is used, for example, in a digital video decoder and the like for decoding television video data.
2. Description of the Related Art
A television video signal is composed of a luminance signal (Y signal) representative of brightness, and a color signal (C signal) representative of color. The C signal is also demodulated using a color reference signal called a sub-carrier for conversion into a Cr signal and a Cb signal. For processing video data, the Y signal and C signal are separately processed independent of each other. Therefore, for realizing a high image quality, the Y signal and C signal are desirably transmitted separately, and the C signal is desirably separated into the Cr signal and Cb signal for separate transmission. However, since a transmission through a single transmission line provides a higher transmission efficiency and easier handling, a high demand exists for the technique for sending a mixture of the Y signal and C signal. Thus, the YC separation technique for separating a composite signal composed of a Y signal and a C signal is also very important.
Known YC separation techniques are based on two types of frequency filters, a two-line comb filter, a three-line comb filter, and the like.
(1) YC Separation Based on Two Types of Frequency Filters:
In a television video conforming to the NTSC (National Television Standards Committee) standard, the C signal has a center frequency at approximately 3.58 MHz. The Y signal in turn uses in principle a frequency band lower than the C signal (see FIG. 1A). When the frequency of the Y signal does not overlap the frequency of the C signal, the YC separation can be achieved only using two types of frequency filters. For example, the C signal can be separated from the composite signal using a bandpass filter which passes therethrough a frequency component near 3.58 MHz, and the Y signal can be separated from the composite signal using a trap filter for attenuating a frequency component near 3.58 MHz. Also, after separating the Y signal with the trap filter, the Y signal may be subtracted from the composite signal to separate the C signal.
(2) YC Separation Based on Two-Line Comb Filter:
At boundaries at which a video changes in the extreme, the frequency of a Y signal may overlap the frequency of a C signal (see FIG. 1B). A two-line comb filter is known to have the ability to remove the influence of such overlapping. In the television video, the position of pixels shifts by one-half period between two sequential horizontal lines (see FIG. 2A), and therefore, the phase of the C signal shifts by one-half wavelength (see FIG. 2B). On the other hand, the phase of the Y signal is the same. Therefore, when there is a correlation between these two horizontal lines (i.e., video data matches), the difference between composite signals on both lines is twice as much as the C signal. The two-line comb filter takes advantage of this principle to separate a C signal from a composite signal. Then, the C signal thus acquired can be subtracted from the composite signal to separate a Y signal.
(3) YC Separation Based on Three-Line Comb Filter:
When there is a correlation among three sequential horizontal lines, the YC separation can be achieved using a three-line comb filter. The three-line comb filter comprises a pair of two-line comb filters, and a processor. One of the two-line comb filters separates a C signal using a horizontal line subjected to the YC separation, and the preceding horizontal line. The other two-line comb filter separates a C signal using the horizontal line subjected to the YC separation, and the following horizontal line. Then, the processor calculates an average value of these two C signals.
When the two-line comb filter or three-line comb filter is used as described above, it is necessary to determine the correlation between two or three sequential horizontal lines. Generally, the sum of C signals on these horizontal lines is used for the determination of the correlation. Specifically, rough C signal values are found from composite signals of sequential horizontal lines using a bandpass filter as mentioned above, and it is determined that there is a correlation between both lines when the sum of these values is equal to or less than a predetermined value. When no Y signal is mixed in the C signal, the sum of C signals on two sequential horizontal lines are zero (see FIGS. 2A, 2B). Even when the Y signal is mixed in the C signal, a rough correlation can be known for the C signal. However, this method can determine that there is a correlation even though a small color difference exists between horizontal lines. When a correlation is erroneously determined, the C signal will be mixed in the Y signal when the C signal is subtracted from the composite sinal to separate the Y signal. The Y signal mixed in the C signal makes dot interference more likely to occur. The dot interference is the phenomenon of displaying small dots in regions in which colors are discontinuous.
Also, when neither the two-line comb filter nor the three-line comb filter is used, a Y signal may be mixed in a C signal. The Y signal mixed in the C signal makes cross color more likely to occur. The cross color is the phenomenon of displaying small stripe patterns.
For preventing the dot interference and cross color, Japanese Patent Kokai No. 10-174119 (Patent Document 1) and No. 2003-299119 (Patent Document 2) are known.
A YC separator circuit of Patent Document 1 relies on a Y signal to determine the correlation. Then, this YC separator circuit multiplies the result of processing indicative of a correlation by a predetermined weighting coefficient to facilitate a determination of the presence of the correlation. In other words, this YC separator circuit intentionally increases the probability of determining that there is a correlation to prevent the cross color.
A YC separator circuit of Patent Document 2 separates a C signal from a composite signal using a bandpass filter, and further removes a burst amplitude from the C signal to prevent the dot interference and cross color.
However, the YC separator circuits disclosed in Patent Documents 1, 2 fail to sufficiently prevent the dot interference and cross color.